This invention relates to a swivel assembly for a downhole tool string. For more than 50 years proposals for transmitting an electronic signal along the length of the tool string used in the exploration and production of hydrocarbons and subterranean energy sources have met limited success for reasons associated with numerous technological obstacles; namely transmitting reliable signals across the joints of the downhole tool string. One successful system disclosed in U.S. Pat. No. 6,670,880 to Hall, which is herein incorporated by reference; introduces a system for transmitting data through a string of downhole components. First and second magnetically conductive and electrically insulating elements are included in both ends of the components. Each element includes a trough with an electrically conducting coil located inside. An electrical conductor connects the coils. Varying currents applied to this system are transformed into magnetic fields which can cross the joints of the downhole components and transform back to varying currents.
A top-hole swivel is a key surface-mounted drill rig component in the tool string for such a system as described in the ″880 patent. It functions as the entry point for drilling fluids and as a load transfer point between the drill string and rig. In some cases it has been used as a data gathering point for logging the well and for monitoring drill string dynamics. When adapted for signal transmission, the swivel becomes the interface between the outside world and the system for transmitting data integrated into the downhole tool string.
Several apparatuses in the art are capable of transmitting data from a tool string to equipment located on the surface. U.S. Pat. Nos. 6,018,301 and 6,075,461 disclose an apparatus for communicating information between downhole equipment and surface equipment. The electromagnetic signal repeater apparatus comprises a housing that is securably mountable to the exterior of a pipe string disposed in a well bore. The housing includes first and second housing subassemblies. The first housing subassembly is electrically isolated from the second subassembly and is secured thereto with a nonconductive strap. The second housing subassembly is electrically coupled with the pipe string and is secured thereto with a conductive strap. An electronics package and a battery are disposed within the housing. The electronics package receives, processes, and retransmits the information between the downhole equipment and the surface equipment via electromagnetic waves.
Another system for transmitting data from a drill string to surface equipment is disclosed in U.S. Pat. Nos. 6,144,316 and 6,177,882. A downhole communications system includes an electromagnetic-to-acoustic signal repeater for communicating information between surface equipment and downhole equipment. The repeater comprises an electromagnetic receiver and an acoustic transmitter. The receiver receives an electromagnetic input signal and transforms the electromagnetic input signal to an electrical signal that is inputted into an electronics package that amplifies the electrical signal and forwards the electrical signal to the transmitter that transforms the electrical signal to an acoustic output signal that is acoustically transmitted.